1. Field of the Invention
The present invention relates to a battery state monitoring circuit for monitoring a battery state, and to a battery device.
2. Description of the Related Art
A conventional battery state monitoring circuit and a conventional battery device are described. FIG. 3 illustrates the conventional battery state monitoring circuit and the conventional battery device.
A battery state monitoring circuit 10 monitors each voltage of batteries 21 to 23 inside the battery device. For a case where two or more batteries are connected, constant current circuits 15 and 16 are provided so as to detect the disconnection of the battery from a connection terminal. For example, in the case of disconnection from an intermediate terminal 12, the constant current circuit 16 causes the intermediate terminal 12 to have the same voltage as a ground terminal 14. Accordingly, a voltage difference between a power supply terminal 11 and the intermediate terminal 12 exceeds an overcharge voltage of the battery 21, and hence an overcharge detection circuit 24 detects the overcharge to output a detection signal. Upon receiving the detection signal, a control circuit 20 turns OFF a charge control transistor 27 to stop the charge (see, for example, Japanese Patent No. 3581428).
The technology disclosed in Japanese Patent No. 3581428 is, however, incapable of stopping the charge in the case of disconnection from the power supply terminal 11 or the ground terminal 14. If the battery is disconnected from the power supply terminal 11, the power supply terminal 11 has the same voltage as the intermediate terminal 13 because of the constant current circuit 15. Therefore, the overcharge detection circuit 24 cannot detect the overcharge to stop the charge. If the battery is disconnected from the ground terminal 14, the ground terminal 14 has the same voltage as the intermediate terminal 12 because of the constant current circuit 16. Therefore, the overcharge detection circuit 26 cannot detect the overcharge to stop the charge.
Further, even if the battery is disconnected from the intermediate terminal 12 or 13, unless a total voltage of the batteries 21 to 23 reaches the overcharge voltage, the overcharge detection circuit 24 or 26 cannot detect the overcharge to stop the charge.